The invention is directed to a method for the modulation of a modulation carrier and to a mobile communication system.
Mobile communication systems as digital radio systems are known, for example, from M. Mouly, M.-B. Paulet, xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d, 1992. The transmission channels on a radio interface between two or more communications parties are variable in mobile communication systems and are subject to many influences, so that the quality of the transmission conditions can change fast.
For example, digital phase-shift keying modulations as described in M. Mouly, M.-B. Paulet, xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d, 1992, particularly pp. 249-259, are employed for modulation.
A digital phase-shift keying (PSK) is a modulation of a modulation carrier with amplitude and frequency kept constant, whereby every characteristic state of a discrete signal corresponds to a specific phase position of the modulation carrier. The transition from one phase position to the other can be a continuous or discontinuous modification. Further, MSK (minimum shift keying) modulation is also known, this being a specific form of CPFSK modulation (continuous phase frequency shift keying). CPFSK modulation is an FSK (frequency-shift keying) modulation given phase-continuous transition between the frequencies.
MSK modulation is a CPFSK modulation with a modulation index of that yields a compromise between noise resistance and bandwidth requirement given orthogonal signals and minimum frequency boost. MSK modulation, however, requires what is sometimes an unacceptably high transmission bandwidth or yields too low a data transmission rate, which can also often not be adequately improved by the smoothed version, for example the known GMSK modulation (Gaussian minimum shift keying).
It must be said that an incomplete bandwidth utilization of the transmission channel exists in mobile communication systems wherein the signals to be transmitted are modulated on a modulation carrier with such a modulation.
Given, for example, a comparison of digital mobile radiotelephone networks in a combined time-division multiple access (TDMA)/frequency-division multiple access (FDMA) as employed, for example, in the GSM (global system for mobile communication) mobile radiotelephone system to a code-division multiple access method (CDMA), a better bandwidth utilization of the CDMA networks is often cited, whereby what is understood by bandwidth utilization is the number of channels per available bandwidth and radio cell. In practice, however, a high bandwidth utilization is difficult to realize in CDMA networks, among other things because of the required synchronization. U.S. Pat. No. 4,495,619 discloses a modulation method for a satellite communication system wherein the desired number of steps are selectable.
An object of the invention is to specify measures for increasing the bandwidth utilization in a mobile communication system.
In general terms the present invention is a method for the modulation of a modulation carrier, whereby a modulation form that is variable in view of its number of steps is provided for a digital transmission channel and the number of steps of the modulation is adaptively matched during operation to the respectively existing transmission conditions. A set of the steps in a modulation with a specific number of steps is a sub-set of the set of steps with the next-higher number of steps.
A training sequence that does not change in the adaptation of the number of steps is transmitted in the digital transmission channel.
A largely constant error probability is achieved by the adaptation of the number of steps in this transmission channel.
A time-division multiplex time slot is respectively to a plurality of transmission channels. The adaptation of the number of steps of the modulation carrier of a transmission channel matched to the transmission conditions are linked with a modification of transmission time resources of this transmission channel that are made use of such that the data transmission rate of this transmission channel remains essentially constant.
The allocation of time slots to transmission channels matched to the transmission conditions is based on a high plurality of transmission channels.
The form of modulation variable in view of its number of steps is determined such that a set of modulated modulation carriers of all possible signals (bit sequences) is a sub-set of a set with the next higher number of steps given modulation with a specific number of steps.
The present invention is also a mobile communication system for signal transmission over a radio interface between a base station and a mobile station that respectively has at least a modulator and a demodulator upon employment of a multi-step modulation of a modulation carrier. A means for the determination of the transmission conditions are provided in the base station and/or the mobile station. A form of modulation variable in view of its number of steps is provided by the modulator and the demodulator. The modulator and the demodulator adaptively match the number of steps of the modulation in the respective digital transmission channel on the radio interface to the transmission conditions respectively present during operation. The modulator is fashioned such that a set of the steps in a modulation with a specific number of steps is a sub-set of the set of steps with the next-higher number of steps.
Advantageous developments of the present invention are as follows.
With the modification of the number of steps in the respective transmission channel, a largely constant error probability that can be identified by the means for the determination of the transmission conditions is achieved by the modulator and by the demodulator.
The radio interface is fashioned according to a time-division multiplex method and/or a combined time-division multiplex/frequency-division multiplex method.
The adaptation of the number of steps of the modulation carrier of a transmission channel matched to the transmission conditions by the modulator and by the demodulator are linked with a modification of transmission time resources of this transmission channel that are made use of such that the data transmission rate of this transmission channel remains essentially constant.
The allocation of time slots to transmission channels matched to the transmission conditions is based on a high plurality of transmission channels.
The form of modulation variable in view of its number of steps is determined such that the spectral width of the transmission channel is not enlarged given modification of the number of steps.
A modification of the number of steps of the modulation of the respective digital transmission channel is linked such with a modification of the transmission power and is monitored by the means for determining the transmission conditions such that an adequate signal-to-noise ratio given a high number of steps can be achieved by a higher transmission power.
The system is a GSM or GSM-like mobile radiotelephone system. For improving the bandwidth utilization, the transmission quality is monitored at the reception side given a transmission between a mobile station and a base station. Given determination of an adequate transmission quality, a switch is made from the use of a xe2x80x9cfull ratexe2x80x9d traffic channel to a xe2x80x9cburst half ratexe2x80x9d traffic channel wherein, analogous to a xe2x80x9chalf ratexe2x80x9d traffic channel, one-half the time slots is used on average, and the payload data correspond to those of a xe2x80x9cfull ratexe2x80x9d traffic channel due to the provision of a 4-step phase modulation instead of a 2-step phase modulation.
The 4-step phase modulation in the xe2x80x9cburst half ratexe2x80x9d traffic channel is designed such that the occupied bandwidth per carrier frequency is not changed compared to the known two-step GMSK modulation.
Proceeding from a xe2x80x9chalf ratexe2x80x9d transmission, for example given improved voice encoding, and analogous switch given good transmission quality is made from a xe2x80x9chalf ratexe2x80x9d transmission channel to a burst quarter rate transmission channel to be defined with 4-step phase modulation that only occupies every 32nd time slot on average and therefore allows four times the number of transmission channels compared to a xe2x80x9cfull ratexe2x80x9d transmission channel.
A 4-step phase modulation is fashioned such that respectively two neighboring bits in a bit stream . . . dixe2x88x921. di, di+1. . . to be transmitted, for example di and di+1, are combined to form a symbol wn with n=xc2xd, whereby an allocation recited in the following Table applies:
whereby modulation of the phase (xcfx860+xcexa3n(knxcfx86(txe2x88x92nT)) ensues with kn=1xe2x88x92⅔(wnxe2x88x92wnxe2x88x921 modulo 4), whereby             φ      ⁡              (                  t          -          nT                )              =                  ∫                  -          ∞                          t          -          nT                    ⁢                        ∫                      -            ∞                                t            xe2x80x2                          ⁢                                            π                              2                ⁢                T                                      ·                          1                                                                    2                    ⁢                    π                                                  ⁢                b                                              ⁢                      (                                          exp                ⁢                                  xe2x80x83                                ⁢                                  (                                                            t                      xe2x80x3                                                              2                      ⁢                                              b                        2                                                                              )                                            -                              exp                ⁢                                  xe2x80x83                                ⁢                                  (                                      -                                                                                            (                                                                                    t                              xe2x80x3                                                        -                            T                                                    )                                                2                                                                    2                        ⁢                                                  b                          2                                                                                                      )                                                      )                    ⁢                      xe2x80x83                    ⁢                      ⅆ                          t              xe2x80x3                                ⁢                      ⅆ            t                                          with      ⁢              xe2x80x83            ⁢      b        =                            ln          ⁢                      xe2x80x83                    ⁢          2                    /              (                  2          ⁢                      π            ·            B                          )            
is a Gaussian ramp of 0 on xcfx80/2 with BT=0.3 as in the 2-step GMSK modulation, whereby B references the bandwidth and T references the duration of a symbol wn.
In the method for the modulation of a modulation carrier according to the invention, the transmission quality is respectively constantly monitored at the reception side both given a transmission from a mobile station to a base station, what is referred to as the uplink, as well as given a transmission from a base station to a mobile station, what is referred to as the downlink, given employment, for example, in the framework of a mobile communication system. Given an adequate transmission quality, the transmission mode is modified. The receiver respectively initiates the modification of the transmission mode and informs the sender of this via a signaling channel that is usually present.
Such a modification of the transmission mode is inventively undertaken in such a way that fewer time slots per time unit are employed on average for that channel that has an adequate transmission quality. So that same average data rate is thereby respectively retained, the number of steps of the respective modulation method is inventively incremented.
The incrementation of the number of steps of the modulation method is thereby undertaken such that the spectral width of the channel does not increase. The channel spacing in frequency-division multiples thus also remains unchanged. Since fewer time slots per channel are required on average according to the invention, the plurality of connections (channels) that can be transmitted over the radio interface, for example for a mobile communication system, can be increased given the same frequency spectrum.
When, by contrast, the transmission quality in a channel drops, then the number of steps of the modulation method is correspondingly reduced in turn or an increase in the transmission power is initiated.
The adaptation of the number of steps can be especially easily converted when the set of the steps of a number of steps forms a sub-set of the next highest number of steps.